Observation of an exotic insulator to insulator transition upon electron doping the Mott insulator CeMnAsO

A promising route to discover exotic electronic states in correlated electron systems is to vary the hole or electron doping away from a Mott insulating state. Important examples include quantum criticality and high-temperature superconductivity in cuprates. Here, we report the surprising discovery of a quantum insulating state upon electron doping the Mott insulator CeMnAsO, which emerges below a distinct critical transition temperature, TII. The insulator-insulator transition is accompanied by a significant reduction in electron mobility as well as a colossal Seebeck effect and slow dynamics due to decoupling of the electrons from the lattice phonons. The origin of the transition is tentatively interpreted in terms of many-body localization, which has not been observed previously in a solid-state material.


Supplementary Table 2 Average compositions obtained from EDS analysis on powders
samples of CeyMnAsO1-xFx.
It is not possible to determine the level of Ce non-stoichiometry through EDX analysis as the Ce vacancy concentration is too small to be determined accurately.We have synthesised over sixty CeMnAsO0.95F0.05phases.After every synthesis, we have collected a 16-hour high resolution X-ray diffraction pattern and performed Rietveld refinement.DC resistivity results always show that samples that exhibit Ce non-stoichiometry from Rietveld refinement have higher transitions than stoichiometric CeMnAsO0.95F0.05phases.It is also not possible to determine the O/F concentration through EDX analysis but the increase of the Ce-O/F bond length as described below demonstrates that nominal doping has occurred.
Laboratory powder X-ray diffraction patterns revealed that CeMnAsO1-xFx phases with x < 0.075, Ce0.96MnAsO0.95F0.05 and Ce0.97MnAsO0.95F0.05 were single phase and could be indexed on the ZrCuSiAs-type tetragonal unit cell with the P4/nmm space group (Supplementary Fig. 2).A minor impurity phase, CeOF (∼ 1.5%), is observed for x = 0.075, which would suggest the F -concentration is less than 0.075.Attempts to chemically dope CeMnAsO further were unsuccessful, as secondary phases of CeF2 and MnF3 impurities began to emerge at x > 0.075.
This would suggest that the doping limit is x = 0.075 for substitution of F -for O 2-.
Rietveld refinements using X-ray diffraction data show that there is no trend in the unit cell parameters upon increasing x (Supplementary Table 3).However, the clear increase in Ce-O/F with x shows that the nominal doping has been successful.In the superconducting CeFeAsO1-xFx series, the same increase in the Ce-O/F bond length is observed with F -doping.The effect of fluorine doping has been suggested to bring the CeO/F charge transfer layer closer to the conducting layer (As-Fe-As block), facilitating electron charge transfer 1 .The Rietveld refinement fits to the ZrCuSiAs-type tetragonal model are shown in Supplementary Figure 2 for all x.
Supplementary Table 3 Selected refined cell parameters and Ce-O/F bond lengths for the CeMnAsO1-xFx series (x = 0 -0.075) obtained from Rietveld fits against laboratory X-ray diffraction data recorded at room temperature.
x behaviour to Mott three-dimensional variable range hopping (3D VRH) is observed for x > 0.
Below this temperature, transport can be described by phonon-assisted tunnelling of electrons between localised states, so that ρ is defined as ρ = ρ0 exp(T0/T) 0.25 and T0 = λα 3 /kN(EF), where T0 describes the degree of electronic disorder, λ is a dimensionless constant, α -1 is equal to the localisation length and N(EF) is the density of localised states at EF.

Table 4 .
Variation of Eg, T0 and TII with x for the CeMnAsO1-xFx solid solution and CexMnAsO0.95F0.05(Ce deficient).The band gap (Eg (calc)) from the DFT calculations is also shown for x = 0 and x = 0.06.The calculated band gap is a sizable overestimate, which is likely to originate from the limitation of DFT to treat the strongly correlated nature of the quasiparticle band gap in this material.
(a) There was not a large enough temperature range to obtain reliable values of T0 for x = 0.075 and 0.050 (Ce deficient) as a result of the higher TII.